mongooseJS can not populate order with items - node.js

Question

I'm trying to get mongoose populate to work.
I have two models one for orders and the other for the order items.
I'm doing this purely to understand populate.
I did have two files for this but I have now got everything in one file called main.js
main.js creates the 3 items in items model.
I then try to populate the order model with the 3 items.
Output for the query now gives the correct populated result.
That is to say the output from the line
console.log(JSON.stringify(orders, null, "\t");
is now:
[
{
"_id": "55d32e4594db780b1bbb4372",
"__v": 0,
"lines": [
{
"price": 2.4,
"quantity": 5,
"_id": "55d32e4594db780b1bbb436f",
"__v": 0
},
{
"price": 3.7,
"quantity": 7,
"_id": "55d32e4594db780b1bbb4370",
"__v": 0
},
{
"price": 1.2,
"quantity": 3,
"_id": "55d32e4594db780b1bbb4371",
"__v": 0
}
]
}
]
The database however is not populating.
Below is the main.js file
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/mydb');
var Schema = mongoose.Schema;
var OrderSchema = new Schema({
lines: [{type: mongoose.Schema.Types.ObjectId, ref: 'Item'}]
});
var ItemSchema = new Schema({
price: Number,
quantity: Number
});
var Order = mongoose.model('Order', OrderSchema);
var Item = mongoose.model('Item', ItemSchema);
var itemArray = [
{price: 2.4, quantity: 5},
{price: 3.7, quantity: 7},
{price: 1.2, quantity: 3}
];
Item.create(itemArray, function(err) {
if (err) {
console.log('Error creating items: ', err);
}
var order = new Order();
Item.find({}, {_id: 1}, function(err, result) {
result.forEach(function(obj) {
order.lines.push(obj._id);
});
}).exec().then(function() {
order.save(function(err) {
if (!err) {
Order.find({})
.populate('lines')
.exec(function(err, orders) {
console.log(JSON.stringify(orders, null, "\t")); // output is populated the database is still not populating.
});
}
});
} );
});

Answer 1

It appears there is some confusion for what the populate method is for. Essentially populate is mongoose's version of a "join". It allows a query (or a document) to automatically retrieve and "join" any associated documents and returns the modified document(s). Under the hood for every referenced value in a document that is to be populated, the populate method will execute a separate query to fetch those associated documents (generally this is by ID but can be a custom query) and then in memory it will replace the references with the retrieved documents.
It does not alter the documents in the collection at all as this is purely a fetch operation.